Condition (REF ) can be used to design an elastic medium where a one–way propagating elastic wave is trapped at its interface. To show this we start from an isotropic elastic material, which has the following form of stiffness tensor in terms of the bulk \(K\)  and shear \(G\)  moduli [1]}
\(C_{i j k l}=\left(K-\frac{2G}{3}\right)\delta _{ij}\delta _{kl}+2G\delta _{ik}\delta _{jl}.\) 
